1. Field of the Invention
This invention concerns a solar cell and more especially a double-heterojunction solar cell in which photons of the solar spectrum are selectively absorbed in one or the other of the heterojunctions depending on their energy. Spectral response and solar cell efficiency are in this way increased.
The invention also concerns a solar cell mounting device improving the concentration of solar rays and the cell reflectivity.
FIG. 1 represents the energy band diagram of a typical heterojunction between two monocrystalline materials. Light of energy less than E.sub.g1 (bandgap energy of the first material which has the larger bandgap) but greater than E.sub.g2 (bandgap energy of the second material which has the smaller bandgap) passes through the first material which acts as a "window" and is absorbed by the second one, and carriers created within the depletion region and within a diffusion length from the junction edges are collected exactly as in a p-n homojuntion cell. Light of energy greater than E.sub.g1 is absorbed in the first material and carriers created within a diffusion length from the junction edges and within the depletion region of this material are also collected. The advantage provided by a heterojunction over the majority of normal p-n junctions lies in their short wavelength response. If E.sub.g1 is large, the high-energy photons are absorbed inside the depletion region of the second material where the carrier collection is very efficient. If the first material is also thick in addition to presenting a broad bandgap, the cell has a lower series resistance and a higher radiation tolerance than a p-n junction made entirely from the second material.
In a homojunction, the barrier height is: EQU V.sub.d =E.sub.g -(E.sub.C -E.sub.F)-(E.sub.F -E.sub.V) (1)
where E.sub.C, E.sub.V and E.sub.F are the conduction band energy, the valence band energy and the Fermi level in the n- and p-sides of the junction respectively.
In a heterojunction, the barrier height in an n-p cell is given by: EQU V.sub.d =E.sub.g2 +.DELTA.E.sub.C -(E.sub.C -E.sub.F)-(E.sub.F -E.sub.V) (2)
and in a p-n cell by: EQU V.sub.d =E.sub.g2 +.DELTA.E.sub.V -(E.sub.C -E.sub.F)-(E.sub.F -E.sub.V) (3)
where E.sub.g2 is the bandgap energy of the material with a small bandgap. The E.sub.C and E.sub.V discontinuities are given by: ##EQU1## where X.sub.1 and X.sub.2 are the electron affinities of the two materials.
It follows from equations (2) and (3) that the barrier potential V.sub.d of an n-p or p-n heterojunction can be greater than in a homojunction by an amount equal to discontinuity energies .DELTA.E.sub.C or .DELTA.E.sub.V if these quantities are positive. In fact, .DELTA.E.sub.C and .DELTA.E.sub.V can be either positive or negative as indicated by equations (4). However, the output power from a heterojunction is no greater than that obtained from a homojunction made in the low bandgap-energy material alone since a high barrier potential V.sub.d is accompanied by a reduced photocurrent. The advantages of a heterojunction do not lie in an increased power output but rather in the elimination of surface recombination and dead-layer problems, the reduction in the series resistance and the increase in radiation tolerance.
2. Description of the Prior Art
U.S. patent application Ser. No. 553,850 filed Feb. 27, 1975 now U.S. Pat. No. 4,017,332, in the name of Lawrence W. JAMES has disclosed a photovoltaic cell comprising (i) a first epitaxial layer of semiconductive material comprising a first given combination of elements selected from columns IIIA and VA of the Periodic Table, a portion of said layer, starting from the bottom surface thereof, having a given conductivity type, the remaining portion of said layer comprising the portion of said layer adjacent the upper surface thereof, having a conductivity type opposite to said given type and such that a rectifying p-n junction is formed in said layer parallel to the upper and lower surfaces thereof, said layer having a given bandgap within the range of 0.4 to 2.3 eV and a given lattice constant within the range of 5.4 to 6.1 Angstrom units; (ii) a second epitaxial layer of semiconductive material joined to the upper surface of said first epitaxial layer and comprising a second given combination of elements selected from columns IIIA and VA of the periodic table, said layer having the said opposite conductivity type, said layer having a given bandgap within the range of 0.4 to 2.3 eV, said layer having substantially the same lattice constant as that of said first epitaxial layer, said second epitaxial layer being joined to the upper surface of said first epitaxial layer so as to form a first heterojunction of like conductivity with the upper portion of said first epitaxial layer; and (iii) a third epitaxial layer of semiconductive material comprising a third given combination of elements selected from columns IIIA and VA of the Periodic Table, a bottom portion of said layer, starting from the bottom surface thereof, having the said given conductivity type, the remaining portion of said layer having said opposite conductivity type and such that a rectifying p-n junction is formed at said layer parallel to the upper and lower surfaces thereof, said layer having a given bandgap within the range of 0.4 to 2.3 eV and higher than the bandgap of said first epitaxial layer, said third epitaxial layer being joined to the upper surface of said second epitaxial layer so as to form a second heterojunction of opposite conductivity with said second epitaxial layer, said second and third epitaxial layers thus providing an n-p junction, said third epitaxial layer comprising means providing a substantial short circuit with said second epitaxial layer in the direction of easy current flow across said p-n junction in said first epitaxial layer and opposite to the direction of easy current flow across the n-p heterojunction formed by said second and third epitaxial layers.
In this patent application, the heterojunctions are made of IIIA-VA compound semiconductors. For example, designating by a the lattice constant, a typical heterojunction is the following: ##EQU2## which gives a lattice mismatch .DELTA.a/a of 3.4%. This is a rather great value and as it is known (see "Semiconductor Lasers and Heterojunction LEDs" by Henry KRESSEL and J. K. BUTLER, Academic Press, 1977, page 300) a lattice mismatch greater than about 2% commonly results in uneven nucleation on the substrate and polycrystalline growth.
The distance between dislocations is: EQU L.sub.d =a.sup.2 /.DELTA.a=(5.86).sup.2 /0.2=170 A
The number of dangling bonds per cm.sup.2 is proportional to the reciprocal of the dislocation distance squared EQU 1/(0.017.times.10.sup.-4).sup.2 =3.4.times.10.sup.11 cm.sup.-2
This a low value with respect to 10.sup.8, the number of valence bonds of a perfectly matched cubic structure crystal.
The aforementioned James patent fails to consider the mismatch between the expansion coefficients of the compound semiconductors forming the heterojunctions.
One of the most successful experimental heterojunction cells of recent years is the pGa.sub.1-x Al.sub.x As-nGa As structure described in the book "Semiconductors and Semimetals", Volume II "Solar Cells" by Harold J. Hovel, Academic Press, 1975, pages 138 and 196. This structure is improved by adding a layer of pGaAs between the two semiconductor materials to form the pGa.sub.1-x Al.sub.x As-pGaAs- nGaAs structure which makes it possible to collect carriers over a greater distance compared to the pure heterojunction. FIG. 80 in the aforementioned publication shows the energy efficiency of this structure in AMO (no atmosphere) and in AMI (at the Earth's surface) as a function of the depth of the pGaAs layer.